This invention relates to a multiphase heat exchanger that provides radial thermal heat transfer between a plurality of individually contained fluids.
Numerous operations are performed on oil and gas wells which require large volumes of nitrogen gas or other cryogenic fluids. These operations may be performed on both onshore and offshore wells. Such operations include foam fracturing operations, acidizing services, jetting down the tubing or down the tubing-casing annulus, nitrogen cushions for drill stem testing, pressure testing, insulation of the tubing-casing annulus to prevent such problems as paraffin precipitation, jetting with proppant for perforating and cutting operations, reduction of density of well workover fluids, displacement of well fluid from tubing during gun perforation operations to prevent excess hydrostatic pressure in the hole from pushing perforation debris into the formation, placing corrosion inhibitors by misting the inhibitor with nitrogen, extinguishing well fires, and other operations. Other operations that require cryogenic fluids at an ambient temperature include pipeline and vessel purging operations and refinery operations such as, recharging catalysts.
Nitrogen is typically stored in its liquid state because of the volume used however, liquid nitrogen will damage most carbon steel pipes used in oil and gas wells. Thus, various heating systems have been developed to raise the nitrogen to an ambient temperature. Typically, 185 BTUs per pound of nitrogen are required to heat the nitrogen to an ambient temperatures of 70xc2x0 F.
One particular such operation is the fracturing of a subsurface formation of the well by pumping a fluid under very high pressure into the formation. The fracturing fluid which is pumped into the well often comprises a foamed gel which is produced by the use of nitrogen gas. The nitrogen for the foam fracturing operation is generally stored in a fluid form at temperatures of approximately xe2x88x92320xc2x0 F.
At pressures encountered in these foam fracturing operations, the nitrogen changes state from a liquid to a gas at approximately xe2x88x92200xc2x0 F. It is, therefore, desirable to heat up the nitrogen gas to a superheated state so that the foam fracturing fluid being pumped down the well will be at an essentially ambient temperature. This is because of the numerous adverse affects upon mechanical equipment of very low temperature which would otherwise be presented by the nitrogen foam.
With regard to land based wells, the nitrogen heating equipment generally includes open flame heaters. A problem is however, presented when performing foam fracturing operations on offshore wells. For safety and environmental reasons, open flames are generally not allowed on an offshore drilling platform. Therefore, it is necessary to provide a heater for the nitrogen which does not have an open flame.
Such flameless nitrogen heaters have previously been provided by utilizing the heat generated by an internal combustion engine and mechanical components driven thereby to heat a coolant fluid which transferred that heat to the nitrogen through a coolant fluid-to-nitrogen heat exchanger.
Numerous problems are encountered with prior art devices mainly because of the use of air as a heat transfer medium. Air is a notoriously poor heat transfer medium as compared to a liquid and the use of ambient air causes the system to be dependent upon ambient air conditions for proper operation. Additionally, due to the large bulky nature of the plenum chamber required for the use of air as a heat transfer medium, the air systems are typically very bulky and heavy. Therefore, there is a need for a flameless nitrogen unit that is compact in size, efficient in the heat transfer process, and economical.
The present invention provides a heat exchanger that collects heat from three sources generated by a drive system using an internal combustion engine and uses the heat to warm a fluid stream. The heat exchanger has three preferably cylindrical chambers one inside the other in coaxial alignment. The second chamber is positioned inside the first, and the third chamber is inside the second. Helical tubes are positioned inside the chambers to carry fluids for the heat exchange process. A first portion of a first helical tube is positioned inside the second chamber, and the second portion of the first helical tube is positioned in the third chamber. The second chamber has a hole in the wall to allow the passage of the second portion of the first tube into the third chamber. A second helical tube is positioned inside the first chamber.
Preferably, the first tube has a first fluid flow therethrough such as nitrogen or some other cryogenic fluid. The second tube has a second fluid flow therethrough, such as hydraulic fluid from the drive system. The second chamber has a third fluid flow therethrough, such as engine coolant, that also flows through the first chamber. Finally, the third chamber has a fourth fluid flow therethrough, such as exhaust from the engine. Preferably, the walls of the second and third chambers are made of a heat conducting material such as stainless steel or copper, so that the fluids flowing therethrough can benefit from radial heat transfer from one chamber to the next.
Alternatively, a third helical tube may be positioned inside the first chamber in parallel with the second helical tube. The third helical tube has a fifth fluid flow therethrough such as hydraulic fluid from the casing of the various pumps used in the drive system. In addition, the fifth fluid flow is preferably in a counter-flow relationship with the fluid flow inside the first chamber.
In a preferred embodiment, all of the fluids traveling through the heat exchanger are in a counter-flow relationship, such that, i.) the first fluid flow through the first portion of the first tube is in a counter-flow arrangement with the third fluid flow in the second chamber, ii.) the first fluid flow through the second portion of the first tube is in a counter-flow arrangement with the fluid flow in the third chamber, and iii) the second fluid flow through the second tube is in a counter-flow arrangement with the fluid flow in the first chamber.
In order to maximize the available surface area, the individual chambers of the heat exchanger may be equipped with spiral fins. The fins are positioned such that they spiral in the same orientation as the helical tubes within the chambers and they are positioned between the turns of the helical tubes. The spiral effect of the fins causes the fluid flow through the individual chambers to come into contact with all sides of the helical tubes. The fins can be positioned between each turn or some of the turns of the helical tubes.
The fourth fluid flow or exhaust passes through the third chamber and exposes the second portion of the first tube to the heat from the exhaust. The exhaust then exits through one end of the heat exchanger. In order to direct the flow from the exhaust, a cylindrical core with spiral fins may be positioned inside the third chamber. The cylindrical core acts to reduce sparks from the exhaust and disperse the flow of the exhaust gases to maximize the surface area of the tube exposed to the exhaust. The unit may also be equipped with a diffuser at each end and a cone shaped inlet and outlet for the exhaust gases to reduce engine noise.
Auxiliary pumps may be used to conduct the fluids through the helical tubes and the coolant through the chambers to compensate for the pressure drop that is incurred as the fluids flow through the heat exchanger.
In another embodiment, a system is provided for converting a liquid to a gas. The system includes a liquid source such as liquid nitrogen or other cryogenic fluid, a drive system including a pump, such as a triplex nitrogen pump connected to the liquid source. The drive system includes an internal combustion engine such as a diesel engine from Detroit, Caterpillar or other commercially available source for driving a hydraulic pump that provides hydraulic fluid to run a hydraulic motor that drives the nitrogen pump in addition to other pumps required to transport the fluids through the system. A heat exchanger is used for providing a first fluid flow, such as water, from the engine in heat exchange relationship with the liquid to heat and convert the liquid to a gaseous state. The heat exchanger further provides a second fluid flow, such as exhaust from the engine for further heating the cryogenic gas in a heat exchange relationship. The heat exchanger further provides a third fluid flow, such as hydraulic fluid from the drive system for heating the first fluid flow. The heat exchanger is designed such that the first fluid flow, second fluid flow, third fluid flow do not mix. Preferably, a fourth fluid flow, comprising casing hydraulic fluid from the pumps and motors, is provided in a heat exchange relationship with the first fluid flow.
It is preferred that the liquid is nitrogen, the first fluid flow is substantially water, the second fluid flow is substantially exhaust, and the third fluid flow is substantially hydraulic fluid, however other fluids may be substituted to achieve specific heat transfer goals of a particular system. A valve element may be positioned in communication with the gas flow as it exits the heat exchanger to control the temperature of the gas as it exits the heat exchanger. The heat exchanger is designed to provide maximum heat transfer with minimum heat loss to the atmosphere, to increase efficiency and lower the cost of providing cryogenic fluids at an acceptable temperature.